This invention relates to techniques for adjusting the pH of post accident containment water in nuclear power plants, and in particular passive techniques.
Nuclear power plants are generally provided with reactor containment vessels that surround the nuclear reactor core and steam generators attached to the core. Water, either pressurized or boiling, circulates through the core, extracting heat from the core for the generation of power. Following an accident that involves the loss of reactor coolant, an emergency core cooling system (ECCS) fills the reactor core with water to a level such that the core is covered. It is possible for the core to suffer some damage in such an accident, and, in the case where the safety injection system fails, to suffer significant damage.
After an accident, radioactive iodine present in the containment vessel can change from a particulate form, such as CeI salts, to a more volatile organic form. The organic form of iodine behaves like a gas and is more likely to leak out of the containment vessel, thereby resulting in greater releases of radioactivity from the plant. Adjusting the pH of the water in the containment vessel prevents the radioactive iodine from changing into the organic form. Therefore, during and after such accidents, it is necessary to adjust the pH of the water that is in the containment sump in order to limit the amount of radiation released from the plant.
Initially after an accident, the pH of the water in the containment vessel may be as low as 3. In order to prevent the formation of significant amounts of organic iodine the pH must be raised to a value of about 7. In current plants, the pH of the post accident water is adjusted by the addition of chemicals such as sodium hydroxide. This is typically accomplished by the use of containment spray pumps to inject sodium hydroxide along with the spray water from the ECCS. This provides an effective means of adjusting the pH. However, this means of injection is subject to some of the same failure mechanisms that can cause the failure of the safety injection system. Some of the common cause failure mechanisms include the complete loss of all AC power, the complete loss of all cooling water, or the complete loss of all ventilation systems.
In some recent advanced nuclear reactors, pumps are not used in the safety systems, and in some passive plants there is no spray system at all. Therefore, a different means of adjusting the pH is required.
One approach is to place water permeable sacks of NaOH crystals in the ECCS sump. When the sump is flooded, the NaOH dissolves in the water. However, this passive system suffers serious drawbacks. First, the NaOH is very caustic and must be isolated from equipment. Second, accidental water spills in the containment vessel could activate this type of passive system, necessitating expensive clean-up procedures to remove the NaOH and to repair any damaged equipment. Third, the sacks must be checked for leaks and replaced frequently. Therefore, it would be advantageous to provide a passive pH adjustment system that overcomes the above stated problems.